Communications satellites have in the past typically employed several antenna subsystems for receiving and transmitting signals from and to the earth respectively. These antenna subsystems are often mounted on a "despun" platform of the satellite so as to maintain a constant antenna orientation relative to the earth. The antenna subsystems may be either fixed or steerable and may operate on different polarizations. For example, one known type of antenna subsystem includes a pair of primary reflectors mounted in aligned relationship to each other, one behind the other. One of the reflectors is vertically polarized and is operative to reflect one of the transmit and receive signals. The other reflector is horizontally polarized and is operative to reflect the other of the transmit and receive signals.
Because of space constraints in communication satellites, the antenna systems for such satellites must be as compact and utilizes a few components as possible. To partially satisfy this objective, imaging reflector arrangements have been deviced to form a scanning beam using a small transmit array. These arrangements achieve the performance of a large aperture phased array by combining a small phased array with a large main reflector and an imaging arrangement of smaller reflectors to form a large image of a small array over the main reflector. An electronically scannable antenna with a large aperture is thus formed, using a small array. One important feature of this imaging arrangement is that the main reflector need not be fabricated accurately, since small imperfections can be corrected efficiently by the array.
In order to provide a compact antenna system, so called quasi-optical diplexers have been employed in the past to separate coincident radio signals of different frequency bands, e.g. a transmit signal and a receive signal. A compact imaging arrangement employing a quasi-optical diplexer of the type discussed above is disclosed in "Imaging Reflector Arrangements to Form a Scanning Beam Using a Small Array", C. Dragone and M. J. Gans, The Bell System Technical Journal, Volume 5, No. 2, Feb. 9, 1979. This publication discloses a frequency diplexer positioned between a transmit array and an imaging reflector. The receive array is positioned on one side of the diplexer, opposite that of the transmit array. Signals in the transmit band pass from the transmit array through the diplexer to the imaging reflector. The diplexer is reflective of signals in the receive band, consequently, a signal in the receive band which is incident on the diplexer is reflected onto the receive array.
With the increasing cost of placing a communications satellite in geosynchronous orbit, it has become increasingly important for the satellite to handle a maximum number of channels, and if possible, different types of communications services. The present invention is directed toward achieving these objectives.